1. Field of the Invention
The present invention relates to a plasma processing method of generating a plasma using two high-frequency powers having different frequencies. The present invention also relates to a method of automatically controlling the beginning of discharge by controlling the power application procedure in beginning discharge using a predetermined sequence. The present invention is effectively applied to mass production of photovoltaic devices based on a roll-to-roll scheme.
2. Related Background Art
As is conventionally known, plasma processing such as CVD, etching, or ashing is performed by supplying a microwave or VHF into a vacuum chamber and simultaneously applying an RF bias using an RF to a substrate or the space in the vacuum chamber to generate a plasma.
As such plasma processing, a deposition film forming method based on microwave plasma CVD using an RF bias is applied to manufacture a photovoltaic device. For example, Japanese Laid-Open Patent Application No. 6-51228 discloses, as a means for beginning microwave plasma discharge, a method of introducing a raw material gas into a discharge area while keeping a low pressure, and applying both a microwave power and an RF power to the discharge area to begin discharge. In this prior art, the RF power serves as a bias on the plasma obtained from the microwave to improve the quality of the deposition film. As compared to a DC bias, the RF bias is less likely to generate spark. Since a higher-frequency energy can be applied to the plasma, a high-quality deposition film can be obtained.
In recent years, VHF plasma CVD using a high frequency in the VHF band has also been examined. As is described in, e.g., Japanese Laid-Open Patent Application No. 7-245269, according to the VHF plasma CVD, a deposition film can be uniformly formed on a substrate having a relatively large area at a high process rate. In this frequency band, the raw material gas utilization efficiency is higher than that in RF plasma CVD, so the deposition film forming rate can be increased. Additionally, since VHF discharge widens the range of discharge conditions (pressure, power to be applied, raw material gas composition, and the like) as compared to microwave discharge, the degree of freedom in controlling the quality of the deposition film increases. VHF plasma CVD is suitable for deposition of an amorphous silicon film. Especially, in depositing a micro-crystallized silicon film, the VHF plasma CVD with the large degree of freedom for film forming conditions can be used to optimize the crystal particle diameter or crystal grain boundary structure, so a high-quality deposition film can be expected.
As a method of manufacturing a large-area device using microwave plasma CVD or RF plasma CVD, a photovoltaic device forming method and apparatus using a roll-to-roll scheme are disclosed in U.S. Pat. No. 4,400,409 or Japanese Laid-Open Patent Application No. 3-30419. In these apparatuses, a plurality of glow discharge areas are arranged. A sufficiently long band-shaped substrate having a desired width is continuously carried in the longitudinal direction of the band-shaped substrate along the route sequentially extending through the glow discharge areas, thereby continuously forming devices each having a semiconductor junction.
Japanese Laid-Open Patent Application No. 7-41954 discloses a method of automatically restoring plasma discharge upon detecting the stop of plasma discharge in a microwave plasma processing apparatus using no RF bias. In this method, the discharge stop state is detected by monitoring the pressure in the discharge space, the reflected wave power in discharge, the potential in the discharge space, or the current value in the discharge space. To restore discharge, a microwave power larger than the discharge keeping power is applied, or the pressure in the vacuum chamber is made lower than the discharge keeping pressure.
However, in automatically beginning/keeping plasma discharge, the following problems are posed.
(1) When plasma discharge is to be begun first using a first high-frequency power, the first high-frequency power stronger than in film formation must be applied to the discharge area while applying a second high-frequency power with an appropriate magnitude. If the order of power application or the timing of adjusting the impedance matching of the second high-frequency power is inappropriate, spark may be generated in the discharge area, or the impedance matching may shift to disappear discharge. This makes it difficult to smoothly begin discharge. In fact, the beginning of discharge depends on operator's experiences, and the procedure of beginning microwave plasma discharge with high reproducibility is indefinite. For this reason, an automatic apparatus can hardly be designed. PA0 (2) If discharge disappears during film formation due to some reason after beginning plasma discharge using the first high-frequency power, discharge is sometimes difficult to begin again because of the reason of (1). Since the rebeginning of discharge takes time, defects occur in the deposition film, resulting in a decrease in yield. PA0 (3) If a large deposition film is uniformly formed by applying the first and/or second high-frequency power from a plurality of application means to one discharge area, discharge by one of the application means may disappear.
if discharge partially disappears, the thickness of the deposition film becomes nonuniform or smaller than a desired thickness, resulting in a decrease in yield.
The present invention has another challenge. When a photovoltaic device is to be manufactured using the plasma CVD apparatus of the roll-to-roll scheme, the apparatus becomes bulky because a number of discharge areas are set in a line along the longitudinal direction. In addition, the raw material gas supply system, the exhaust system, and the power supply system tend to be complex. To improve the reproducibility of the device performance or operability of the apparatus, the operation of the apparatus is preferably automatized as far as possible. If devices are to be mass-produced to reduce the manufacturing cost, it is essential to automatically operate the entire apparatus to improve the operating efficiency or yield.